Professor Vijay Gupta - http://gupta.seas.ucla.edu/
SUMAMRY
UCLA researchers in the Department of Mechanical and Aerospace Engineering have developed a fast, reliable, atomic-level sensitive adhesion metrology tool for measuring the tensile strength of multilayer interfaces and epoxy-bonded joints such as those appearing in the semiconductor (ICs, electronic packages), aerospace (composite joints), automotive (metal joints), and paint industries. The method can be used to initiate a subcritical crack to measure the actual strength of an interface, in-situ, in a semiconductor device, or it can be repeatedly used to further grow the initially created flaw to determine the strength of the initially-created flaw or equivalently its fatigue strength. Fundamentally, each stress pulse measures the actual strength of the flawed interface created by the previous pulse. The technique can be used for quick optimization of a manufacturing process or used as a quality-control pass/fail test for a manufactured device by comparing the laser energy for flaw initiation with a pre-determined benchmark. The technique can also allow the use of laser spallation rather than wet-etching to separate thin films from substrates.
This technique enables the production of ever thinner line widths for semiconductor and nanocircuitry applications on any substrate by transferring nanofilms that can only be grown on specific substrates. This can be important as the current use of lithography for semiconductor circuitry is reaching its limit. The two inventions are fairly separate-one allowing development and quality control of a semiconductor process by evaluating the tensile strength or fatigue strength of a semiconductor device through in-situ testing, while the other allows actual fabrication of a high performance device on a variety of substrates, including plastic.
BACKGROUND
For interface strength measurement of thin films, the current state of the art of laser spallation (as described in US Patent 5,438,402, by the current inventor) does not permit the spallation and measurement of films less than 1 micron. The UCLA innovation extends the capability to test films below 0.5 microns in thickness not only in their planar form as the predecessor technology does, but extends it to measure the strengths, in-situ, in a semiconductor device with their actual multiplayer 3D non-planar geometries. The technique can be used either in a single pulse mode to develop or monitor a manufacturing process or it can be used in a multiple pulse mode configuration, with each pulse measuring the strength of the flawed interface-state created by the previous pulse. The second application of separating films and structures from substrate has a major advantage over the wet-etching technique used most widely in the semiconductor industry to separate thin films or their structures from substrates. Laser spallation has the advantage of greater precision and lower cost since it is less dependent on precision alignment of the sacrificial layer and etchants. In addition, damage to small structures during etching can be avoided.
INNOVATION
Laser spallation is effected by a laser pulse focused on a metal film sandwiched between a substrate and a layer of silicon-dioxide. The melting-induced expansion of the metal induces a compressive stress pulse in the substrate, which propagates towards a test coating deposited on its front surface. The wave reflects into a tensile wave from the test coating’s free surface and leads to its complete removal at sufficiently high amplitude. The interface tensile stress is obtained by measuring the transient displacement of the coating’s free surface by using an optical interferometer.
A reduction of thickness of the film is effectuated by modification of the stress wave profile due to glass as compared with the stress wave profile due to silicon or any other engineering substrate. This innovation lies in the chemistry of the glass, understanding of the relationship of the thickness of the specimen compared with the wavelength of the stress wave and new techniques to control the stress wave profile.
STATUS OF DEVELOPMENT
Interface strength in a variety of systems has been measured. These materials include metals, ceramics, and polymeric coatings with engineering substrates. A free surface of glass was coated with a 0.5 micron thick Al film and constrained from the top using a 40-50 micron layer of water glass. TiN/glass and Ni/glass systems, which have interest in display technology, were chosen to demonstrate applicability. Also, a Cu/TiN bilayer system deposited on a silicon wafer was demonstrated. The technique has been demonstrated to generate tensile stress and using that to controllably separate various interfaces, in-situ, within a Si device, electronic substrate, package, and fully assembled motherboards. Of particular interest was to correlate the process variables to optimize the strength and stability of solder joint multilayer interfaces and moisture-related degradation of underfills and other polymer interfaces that are buried in these devices. The technique was also shown to create controlled microcracks and growing them till failure using multiple pulses in various interface systems.
For the second application, nanobumps deposited on specialized substrates were separated using the new waves. The bumps were caught on desired electronics substrates having a very thin layer of adhesive or a self-assembled molecular layer. No restrictions were placed on the type of substrate used. A laser impinged onto the undersurface of a multiplayer plastic substrate containing lead bumps, needed to connect the Si device to communicate with the substrate circuitry. The stress wave separated the bumps and these separated bumps were caught on a flexible plastic tape. It is interesting to note that as the laser energy increased, separation below the bump areas was also accomplished. Thus the bumps along with their underlying films were pulled free from the substrate and caught on the plastic tape. Numerous combinations of substrates and films on the order of 0.5 microns have been separated and caught. The technique thus allows generation of local tensile stress with controlled amplitudes that can be used to initiate and propagate cracks.
PATENT STATUS
United States Of America Issued Patent 7,487,684 02/10/2009